This invention relates generally to a device that interfaces communications network enabled systems with a phone network, and more specifically to an improved direct access arrangement device interconnecting equipment with a phone network.
One example of a data communications network enabled system is a security alarm system. Security alarm systems are utilized in a variety of applications in both residential and commercial environments. Security alarms monitor one or more remote components and, based upon feedback from the remote components, carry out various security and emergency related functions. Security alarm systems typically communicate with one or more remote terminals, such as at a host or central operations terminal, over conventional phone lines maintained within the phone network.
Security alarm systems generally include an alarm panel joined to a modem that provides bidirectional communication over the phone network. The modem conveys security and emergency related data at various connection speeds (e.g. 2400 bps) between the phone network and the security panel. Each phone network operates with a standardized profile for parameters such as line input and output levels, signal attenuation, line impedance and the like. One example of an average US line profile is a line impedance of 600 ohms, a line output level of approximately −23.5 dBm, a line input level of −10 dBm, and a line attenuation of 13.5 dBm. The communication of alarm signals from residential and commercial security alarm systems is typically accomplished using an integrated modem in a security system alarm panel to modulate the signal for transmission over a phone line. Many past and current systems use low bandwidth communications channels. However, due to advancing technology and new features in modern security systems, it has become desirable for some systems to support a higher bandwidth communication channel. As a result, a need exists to improve the on-board modem hardware to achieve better levels of signal integrity. The security device industry is sensitive to product component cost which limits the use of integrated solutions that might otherwise offer higher performance.
Direct Access Arrangement (DAA) devices are used in certain fields, other than security alarm systems, to interface other types of equipment with the phone network. For example, a DAA device may be utilized to terminate a telephone connection at an end user's home phone. In addition, DAA devices have been used with modems to connect personal computers to the phone network.
However, conventional designs for DAA devices are quite limited. Conventional DAA devices utilize a transformer or an optical isolator to isolate the phone network from the end user's phone or PC. Alternative convention implementations involve using a transformer-based DAA or an integrated DAA chip. An integrated DAA chip utilizes high quality optical isolators or a capacitor isolated common-mode barrier, both solutions of which are costly. While transformers are useful in more demanding applications, transformers may become cost prohibitive in certain applications. Optical isolators are more cost effective than transformers. However, optical isolators in general introduce high levels of signal distortion. Security alarm systems have relatively stringent signal distortion requirements. Consequently, the high levels of signal distortion potentially introduced by optical isolators have limited the use of DAA devices with optical isolators in security alarm systems. Conventional DAA devices were generally designed for 300 bits per second (bps) modem operations. Modems that operate at data rates of 300 bps do not experience overly high levels of signal distortion and are relatively tolerant of signal distortion. Thus, signal distortion is not a large concern.
Conventional DAA devices do not adequately account for signal distortion to be used in security alarm systems. Signal distortion may manifest itself as carrier jitter. Consequently, an amount of distortion that creates no problems for low baud rate applications becomes quite problematic for higher baud rate applications due, in part, to the increased amount of jitter. Unlike transmission echo, distortion is not canceled by the telephone device within the phone network, nor by linear filters within the modem.
One purpose of the optical isolator is to transfer electrical signals across an optical path in order to isolate from one another both sides of the electrical device. Optical isolator blocks common mode electrical signals, thereby leaving differential mode electrical signals that are coupled through an optical path. A net result is a transfer of electrical current. A parameter that defines the ratio between the input and output current is known as the current transfer ratio. Low cost optical isolators, such as the LTV-816S-TA1-DSC, are inherently non-linear. Typically, the current transfer ratio is not constant and ranges (e.g. from 0.7 to 1.6 at an input current level of 1 mA to 16 mA). The non-constant current transfer ratio creates the non-linearity exhibited by the optical isolator.
A need remains for an improved DAA device that may be constructed in a relatively cost effective manner while maintaining low signal distortion.